Fe-Catalyzed reactions of 2-chloro-1,7-dienes and allylmalonates David Nečas a, Pavel Drabina b, Miloš Sedlák b and Martin Kotora a,c a Department of Organic and Nuclear Chemistry, and Center for Structural and Synthetic Application of Transition Metal Complexes, Faculty of Science, Charles University, Hlavova 8, 128 43 Prague 2, Czech Republic b Department of Organic Chemistry, Faculty of Chemical Technology, University of Pardubice, Nám. Čes. Legií 565, 532 10 Pardubice, Czech Republic c Institute of Organic Chemistry and Biochemistry, Academy of Sciences of the Czech Republic, Flemingovo náměstí 2, 166 10 Prague 6, Czech Republic Abstract Fe(III)(BIAP)Cl 3 complex catalyzes alkylative cyclization of 2-chloro-1,7-heptadiene in the presence of triethylaluminum. It also catalyzes deallylation of certain 2-allyl-2-substituted malonates under the same reaction conditions. Graphical abstract Although iron is one of the most abundant metals found in nature, the use of its compounds as catalysts for the formation of C C bonds in organic synthesis is rather limited, especially in comparison with other transition metals (Ni, Pd, Rh, etc.). Problems associated with the use of the iron compounds as the catalysts often stem from the fact that the catalytically active species are sensitive to reaction conditions used and often lack generality with respect to the structure of reactants. On the other hand, iron compounds are able to catalyze a plethora of interesting transformations, 1 and are thus attractive targets for further research. In iron-catalyzed cross-coupling reactions, for example, after pioneering work by Kochi in the early 1970s, 2 iron catalyzed processes made a [3] and [4] comeback in the work of Fürstner and Knochel, who showed its generality. We have recently reported a new process: an alkylative cyclization of 2-chloro-1,7-dienes in the presence of a catalytic amount of a Fe-phosphine complex and trialkylaluminums. The main feature of this reaction was an unprecedented iron-catalyzed alkyl group (Et, Me) transfer from the trialkylaluminum to the cyclized products. 5 In addition we observed that under the same reaction conditions certain allylmalonate substrates underwent deallylation. The latter process was later shown to proceed also under Ni- 6 and Rh- 7 catalyses. Because of our interest in the development of new Fe-catalyzed reactions, we decided to search for a better ironbased catalytic system, especially because Fe-phosphine complexes lack generality with respect to the substrate. We envisioned that a change of the phosphorus-based ligand for a nitrogen-based one (BIAP = bis- (imidazolonyl)pyridine) could result in the formation of more powerful catalysts. This assumption was based on
the results obtained during Rh-catalyzed deallylations, in which it was clearly demonstrated that the BIAP ligated complexes are more catalytically active then those with phosphines. 7 At the outset, the starting Fe(III)(BIAP)Cl 3 1a e complexes were prepared according to the previously reported procedure (Fig. 1). 8 As a test reaction we chose alkylative cyclization of 9-allyl-9-(2-choroallyl)-9H-fluorene 2. The reaction was carried out under standard conditions. 9 The results obtained are presented in Table 1. In all cases the reaction was almost complete within 1 h and prolonging the reaction time to 24 h did not result in substantial improvement in the yield of the cyclized product 3. This is in sharp contrast with Fe-phosphine catalysts, where the yield gradually increased with the reaction time. As for the catalytic activity, the best yield of 3 was obtained with catalyst 1a. It was slightly higher when 15 mol % of 1a was used (see Table 2). Figure 1. Fe(III)(BIAP)Cl 3 complexes 1a e. Table 1. Alkylative cyclization catalyzed by 1a e a Catalyst 1 t (h) 3, Yield b (%) 1a 1 35 (53) c 24 48 (61) c 1b 1 11 24 27 1c 1 26 24 38 1d 1 30 24 38 1e 1 26 24 36 a 5 mol % of the catalyst. b GC yields. c 15 mol % of the catalyst.
Table 2. Deallylation catalyzed by Fe(III)(BIAP)Cl 3 complex 1a a Malonate 4 R 5, Yield b (%) 4a 5a, 13 4b 5b, 26 4c 5c, 83 (48) c 4d 5d, 59 (45) c 4e 5e, 38 4f 5f, 91 a 5 mol %; Et 3Al, 200 mol %, 20 C, 24 h. b 1 H NMR yields. c 1 h. Next we turned our attention to the deallylation reaction, because in our previous experiments the Fe-phosphine complex catalyzed the reaction only with allyl(chloroallyl)malonate 4c. 5 Catalyst 1a was used because it had proved to be the most active catalyst in the alkylative cyclization, and reactions were carried out under standard conditions. 10 The deallylation of diallylmalonate 4a proceeded with low conversion giving only a low yield (13%) of allylmalonate 5a together with recovered starting material. Similarly allyl(methallyl)malonate 4b was deallylated and also in low yield giving only 26% of methallylmalonate 5b. On the other hand, the deallylation of allyl(chloroallyl)malonate 4c gave rise to the deallylated product, (chloroallyl)malonate 5c, in high 83% yield (48% after 1 h). Also, the deallylation of butyl(allyl)malonate 4d proceeded to a reasonable extent: butylmalonate 5d was formed in 59% yield (45% in 1 h). The efficiency of deallylation fell again in the case of allylbenzylmalonate 4e, which gave only 38% of 5e. Surprisingly, almost quantative deallylation was observed for allylphenylmalonate 4f: 91% of 5f. Attempts to deallylate diallylcoumaranone failed completely, the starting material remaining intact. In summary, we have shown that Fe(III)-BIAP complexes 1 can catalyzed alkylative cyclization, the catalytic activity of 1a being close to that of Fe-phosphine complexes. 5 As for the deallylation, the reactions carried out in the presence of 1a gave better results than Fe-phosphine complexes. Nonetheless, the results again showed that the course of the reaction is highly dependent on the structure of substrates and is rather unpredictable. The scope of the reaction with respect to other substrates as well as further elaboration of the ligand structure is under investigation. Spectral characteristics ( 1 H and 13 C NMR) of all formed compounds were in agreement with the [6] and [7] previously published data. Acknowledgements
We gratefully acknowledge financial support by the Ministry of Education of the Czech Republic to the Center for Structural and Synthetic Application of Transition Metal Complexes (Project No. LC06070), the Project MSM 002 162 7501, and the Project MSM 002 162 0857. References and notes 1 (a)for reviews, see: C. Bolm, J. Legros, J. Le Paih and L. Zani, Chem. Rev. 104 (2004), pp. 6217 6254. (b)a. Fürstner and R. Martin, Chem. Lett. 34 (2004), pp. 624 629. (c)d. Nečas and M. Kotora, Chem. Listy 100 (2006), pp. 967 973. 2 (a)m. Tamura and J.K. Kochi, J. Am. Chem. Soc. 93 (1971), pp. 1487 1489. (b)m. Tamura and J.K. Kochi, Bull. Chem. Soc. Jpn. 44 (1971), pp. 3063 3073. (c)m. Tamura and J.K. Kochi, Synthesis (1971), pp. 303 305. (d)j.k. Kochi, Acc. Chem. Res. 7 (1974), pp. 351 360. 3 (a)a. Fürstner and A. Leitner, Angew. Chem., Int. Ed. 41 (2002), pp. 609 612. (b)a. Fürstner, A. Leitner, M. Méndez and H. Krause, J. Am. Chem. Soc. 124 (2002), pp. 13856 13863. (c)b. Scheiper, M. Bonnenkessel, H. Krause and A. Fürstner, J. Org. Chem. 69 (2004), pp. 3943 3949. 4 (a)w. Dohle, F. Kopp, G. Cahiez and P. Knochel, Synlett (2001), pp. 1901 1904. (b)c. Duplais, F. Bures, I. Sapountzis, T.J. Korn, G. Cahiez and P. Knochel, Angew. Chem., Int. Ed. 43 (2004), pp. 2968 2970. (c)i. Sapountzis, W.W. Lin, C.C. Kofink, C. Despotopoulou and P. Knochel, Angew. Chem., Int. Ed. 44 (2005), pp. 1654 1658. (d)g. Dunet and P. Knochel, Synlett (2006), pp. 407 410. (e) Lyle, F. R. U.S. Patent 5 973 257, 1985; Chem. Abstr., 1985, 65, 2870. 5 D. Nečas, M. Kotora and I. Císařová, Eur. J. Org. Chem. (2004), pp. 1280 1285. 6 (a)d. Nečas, M. Turský and M. Kotora, J. Am. Chem. Soc. 126 (2004), pp. 10222 10223. (b)d. Nečas, M. Turský, I. Tišlerová and M. Kotora, New J. Chem. 30 (2006), pp. 671 674. 7 M. Turský, D. Nečas, P. Drabina, M. Sedlák and M. Kotora, Organometallics 25 (2006), pp. 901 907. 8 M. Sedlák, P. Drabina, I. Císařová, A. Růžička, J. Hanusek and V. Macháček, Tetrahedron Lett. 45 (2004), pp. 7723 7726. 9 A typical experimental procedure for alkylative cyclization. To a solution of 2 (0.5 mmol, 140 mg) and Fe-BIAP complex 1a (0.025 mmol, 18 mg) in dry toluene (3 ml) under argon was added Et 3Al solution in toluene (2 M, 1 mmol, 0.5 ml). The reaction mixture was stirred under argon for 24 h at 20 C. After that it was quenched with water (1 ml) followed by 3 M solution of HCl (3 ml). The organic layer was separated and dried (MgSO 4). Then the reaction mixture was analyzed by 1 H NMR. 10 A typical experimental procedure for deallylation reaction. To a solution of 4 (0.5 mmol, 140 mg) and Fe-BIAP complex 1a (0.025 mmol, 18 mg) in dry toluene (3 ml) under argon was added Et 3Al solution in toluene (2 M, 1 mmol, 0.5 ml). The reaction mixture was stirred under argon for 24 h at 20 C. After that it was quenched with
water (1 ml) followed by 3 M solution of HCl (3 ml). The organic layer was separated and dried (MgSO 4). Then the reaction mixture was analyzed by 1 H NMR.